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Synlett 2008(13): 2023-2027  
DOI: 10.1055/s-2008-1077974
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Use of Pyridinium Chlorochromate and Reusable Polyaniline Salt Catalyst Combination for the Oxidation of Indoles

Chebolu Naga Sesha Sai Pavan Kumara, Chebrolu Lavanya Devia, Vaidya Jayathirtha Raoa, Srinivasan Palaniappan*b
a Organic Chemistry Division II, Indian Institute of Chemical Technology, Hyderabad 500007, India
b Organic Coatings & Polymers Division, Indian Institute of Chemical Technology, Hyderabad 500007, India
Fax: +91(40)27193991; e-Mail: palani74@rediffmail.com;
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Publication History

Received 26 March 2008
Publication Date:
15 July 2008 (online)

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Abstract

A novel method is described herein for the simple, convenient and efficient oxidation of indoles to isatins using pyridinium chlorochromate with the aid of polyaniline salt catalyst at room temperature or at reflux in dichloroethane. Interestingly, oxidation of 3-alkyl indoles by this procedure gave 3-hydroxy 3-alkyl oxindoles. On the other hand, indol-3-alkanols gave mixtures of isatins and 3-formyl indoles. This is the first example of use of polyaniline as a catalyst in oxidation reaction.

Key words

indoles - oxidation - PCC - isatin - polyaniline

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18

Representative Experimental Procedure: In a typical experiment, PCC (2.5 mmol, 537 mg) was added to a solution of indole (1 mmol, 117 mg) in 1,2-DCE (5 mL). Polyaniline salt catalyst [15 wt% with respect to indole (17 mg)] was added and the reaction mixture was stirred at r.t. or at reflux. Completion of the reaction was monitored by TLC. After completion, the reaction mixture was filtered through a Celite® pad and washed with EtOAc (2 × 5 mL). The combined organic extracts were dried and evaporated under vacuum. The crude product was purified by silica gel column chromatography (n-hexane-EtOAc, 3:1) to furnish isatin (129 mg, 88%). A similar procedure was adopted for the preparation of other isatins, 3-hydroxy oxindoles and the authenticity of the products was confirmed from their ¹H NMR and mass spectral data.

19

Spectral Data for New Compounds: 1-Butyl-5-chloroindoline-2,3-dione (Table 2, entry 8): TLC: R f 0.5 (30% EtOAc-hexanes); orange solid; mp 62-63 ˚C. IR (KBr): 3089, 2953, 1737, 1606, 1448, 1339, 1185, 1116, 844, 720 cm-¹. ¹H NMR (200 MHz, CDCl3): δ = 7.49 (m, J = 8.1 Hz, 2 H), 6.80 (d, J = 8.1 Hz, 1 H), 3.70 (t, J = 7.3 Hz, 2 H), 1.67 (quint, J = 7.3 Hz, 2 H), 1.43 (m, J = 7.3 Hz, 2 H), 0.98 (t, J = 7.3 Hz, 3 H). ¹³C NMR (200 MHz, CDCl3): δ = 182.5, 157.6, 149.4, 137.5, 129.3, 125.1, 118.3, 111.8, 40.07, 29.08, 20.0, 13.5. MS (ESI): m/z = 238 [M + H+]. HRMS (ESI): m/z [M + H+] calcd for C12H13NClO2: 238.0634; found 238.0630.
1-Methyl-5-nitroindoline-2,3-dione (Table 2, entry 9): TLC: R f 0.3 (50% EtOAc-hexanes); yellowish orange solid; mp 203 ˚C. IR (KBr): 2924, 1743, 1610, 1465, 1331, 1289, 1109, 1074, 845, 747, 710, 600 cm-¹. ¹H NMR (200 MHz, CDCl3): δ = 8.47 (dd, J = 8.8 Hz, 1 H), 8.34 (d, 1 H), 7.06 (d, J = 8.8 Hz, 1 H), 3.29 (s, 3 H). ¹³C NMR (200 MHz, CDCl3): δ = 182.1, 157.3, 150.3, 140.5, 128.0, 118.3, 116.7, 111.5, 26.2. GC-MS: m/z = 206 [M+]. Anal. Calcd for C9H6N2O4: C, 52.44; H, 2.93; N, 13.59. Found: C, 52.40; H, 2.78; N, 13.42.
1-Octyl-1 H -pyrrolo[2,3- b ]pyridine-2,3-dione (Table 2, entry 15): TLC: R f 0.5 (30% EtOAc-hexanes); yellow solid; mp 74-76 ˚C. IR (KBr): 3063, 2925, 2855, 1737, 1592, 1456, 1353, 1263, 1085, 783, 738 cm-¹. ¹H NMR (200 MHz, CDCl3): δ = 8.41 (d, 1 H), 7.78 (d, 1 H), 7.05 (t, 1 H), 3.83 (t, 2 H), 1.74 (t, 2 H), 1.25-1.35 (m, 10 H), 0.87 (t, 3 H). ¹³C NMR (200 MHz, CDCl3): δ = 180.9, 163.3, 157.3, 154.9, 131.8, 118.1, 110.9, 38.9, 30.7, 28.6, 28.1, 26.5, 25.8, 21.6, 13.03. MS (ESI): m/z = 261 [M + H+]. HRMS (ESI): m/z [M + H+] calcd for C15H21N2O2: 261.1603; found: 261.1606.